Experts down under say the teenager is the living "holy grail of transplants" are have put together a team to research her against-the-odds transformation. They hope their findings will help other transplant patients and even multiple sclerosis and type-1 diabetes sufferers.

Demi-Lee suffered liver failure and had a liver transplant at the age of nine in 2001.

Several months on from the transplant, her doctors at Westmead Children's Hospital in Sydney were shocked to discover her blood type had changed to match the blood type of her deceased male donor.

On closer inspection, specialists found that stem cells from the donor liver had penetrated her bone marrow, effectively resulting in a naturally occurring bone marrow transplant. It makes Demi-Lee the first person ever recorded whose body has entirely accepted a transplanted organ.

Remarkably, Demi-Lee no longer needs anti-rejection drugs to keep her alive. The drugs, known as immunosuppresants, can have toxic effects on organs and cause severe infections.

Other organ transplant patients have been taken off anti-rejection drugs, but nearly only with the aid of a bone-marrow transplant.

Head of haematology, Dr Julie Curtin, described the phenomenon as a natural bone-marrow transplant: "The holy grail of transplants was achieved.

"That's what we were trying to achieve for everybody, but Demi-Lee's body has done that itself."

Demi-Lee's doctor, Michael Stormon, said: "We were stunned, absolutely stunned, and also very puzzled," said Dr Stormon, who reported the case in Australia's New England Journal of Medicine.

"Even going through the literature and seeking advice internationally we weren't able to find any other cases like it. She is a girl in six billion." But Demi believes she is just like any other normal teenager. Despite having defied modern medicine, she says she is merely happy to finally be a "normal girl" after an ordeal she describes as a nightmare.

The teenager has thanked her doctors at the Children's Hospital - which she calls her "third home" - and her liver donor, a 12-year-old boy who died from a brain injury.

"I'm probably the most grateful person because that has saved my life, that gave me a chance to fulfil my life," she said.

"I just want to live it the most I've got for them and to show them that I'm so grateful."

Demi's mother, Kerrie Mills, described it as "miraculous", and Dr Stormon agreed it was "still highly difficult to explain at this stage".

The challenge for the medical team now is to try to identify how the phenomenon happened and whether it can be replicated.

"That's easier said than done," added Dr Stormon. "We're not sure the reasons behind why this has happened but it may be that a complex range of circumstances have aligned to bring it about.

"Now the task is to find out what those are so we can replicate them and allow other transplant patients to has such a complete acceptance."